That biofuel may not be as green as you think it is

A perspective article in this week's edition of Science looks at a study done …

A while back, I covered an article published in the International Journal of Global Energy Issues that examined the "greenness" of alternative energy sources. This study looked not only at the amount of energy that is gained from an alternative source relative to fossil fuels, but how much energy is used and pollution created during formation, operation, and decommissioning of a source. It found what some may consider shocking results: wind and geothermal energy were true green resources compared to fossil fuels; solar power, on the other hand, is not ecofriendly until economies of scale come into play.

In this week's edition of Science, a feature discusses a recent study examining the total environmental impact from various types of biofuels. The actual study was carried out by R. Zah et al., at the behest of the Swedish government. While nearly all crop-derived biofuels emit less greenhouse gases than fossil fuels, whether or not they are better for the environment is still open for debate. Some studies have suggested that corn-derived ethanol consumes more energy than it produces, others have found a slight net benefit; most studies have found that crops such as switch grass have outperformed both corn and soy.

How well one biofuel performs relative to another can rapidly change when the total impact to the environment is considered. According to the author of the perspective, "a key factor affecting biofuel efficacy is whether native ecosystems are destroyed to produce the biofuels." The example used is that sugarcane becomes much less environmentally friendly if rainforests are being razed to make room for more sugarcane fields. If biodiversity is taken into account, then the scales would tip further away from being truly green.

Another factor that must be considered is trace-gas emissions; corn and rapeseed require nitrogen fertilizers which result in nitrous oxide emissions. If NOx emissions are included in the analysis, then grasses and woody coppice become more favorable, while corn and canola can actually be worse for global warming than burning fossil fuels.

The author of the perspective points out the "apples and oranges" problem that one often encounters when trying to compare different biofuels. They highlight the fact that Zah and coauthors were able to construct an objective evaluation method that used two criteria: greenhouse-gas emissions relative to gasoline, and overall environmental impact—an aggregated measure of natural resource depletion and damage to human health and ecosystems. The authors compared 26 different biofuels as well as gasoline, diesel, and natural gas. 21 out of the 26 biofuels examined emitted less greenhouse gases than gasoline when burned. However, almost half—12 out of 26—had a greater composite environmental cost than fossil fuels. Of those biofuels who had a greater (negative) impact were many economically important ones, including corn ethanol, sugarcane ethanol, and soy diesel. The biofuels that fared the best were those produced from residual products, such as biowaste or used cooking oil.

Matt Ford / Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems.